Packaging containers of the single use disposable type for liquid foods are often produced from a packaging laminate based on paperboard or carton. One such commonly occurring packaging container is marketed under the trademark Tetra Brik Aseptic® and is principally employed for aseptic packaging of liquid foods such as milk, fruit juices etc, sold for long term ambient storage. The packaging material in this known packaging container is typically a laminate comprising a bulk core layer of paper or paperboard and outer, liquid-tight and heat sealable layers of thermoplastic polymers. In order to render the packaging container gas-tight, in particular oxygen gas-tight, for example for the purpose of aseptic packaging and packaging of milk or fruit juice, the laminate in these packaging containers normally comprises at least one additional layer, most commonly an aluminium foil.
On the inside of the laminate, i.e. the side intended to face the filled food contents of a container produced from the laminate, there is an innermost layer, applied onto the gas barrier layer of aluminium foil, which innermost, inside layer may be composed of one or several part layers, comprising heat sealable adhesive polymers and/or polyolefins. Also on the outside of the core layer, there is an outermost heat sealable polymer layer.
Similar packaging containers, but without any gas barrier layer, e.g. aluminium foil, are produced for packaging of liquid food products intended for chilled storage and distribution, i.e. products marketed with a shorter shelf life.
The packaging containers are generally produced by means of modern, high-speed packaging machines of the type that form, fill and seal packages from a web or from prefabricated blanks of packaging material. Packaging containers may thus be produced by reforming a web of the laminated packaging material into a tube by both of the longitudinal edges of the web being united to each other in an overlap joint by welding together the inner- and outermost heat sealable thermoplastic polymer layers. The tube is filled with the intended liquid food product and is thereafter divided into individual packages by repeated transversal seals of the tube at a predetermined distance from each other below the level of the contents in the tube. The packages are separated from the tube by incisions along the transversal seals and are given the desired geometric configuration, normally a cuboid shape, by fold formation along prepared crease lines in the packaging material.
The main advantage of this continuous tube-forming, filling and sealing packaging method concept is that the web may be sterilised continuously just before tube-forming, thus providing for the possibility of an aseptic packaging method, i.e. a method wherein the liquid content to be filled as well as the packaging material itself are reduced from bacteria and the filled packaging container is produced under clean circumstances such that the filled package may be stored for a long time even at ambient temperature, without the risk of growth of micro-organisms in the filled product. Another important advantage of the Tetra Brik®-type packaging method is, as stated above, the possibility of continuous high-speed packaging, which has considerable impact on cost efficiency.
A layer of an aluminium foil in the packaging laminate provides gas barrier properties quite superior to most polymeric gas barrier materials. The conventional aluminium-foil based packaging laminate for liquid food aseptic packaging is the most cost-efficient packaging material, at its high performance level, available on the market today.
Among the efforts of developing alternative cost-efficient packaging materials and minimizing the amount of raw material needed for the manufacturing of packaging materials, should be mentioned barrier polymers suitable for extrusion coating or extrusion lamination, as well as pre-manufactured films from polymers having barrier properties. Further alternatives are pre-manufactured films having multiple barrier functionalities, which may replace the aluminium-foil. Previously known such examples are films combining multiple layers, which each contribute with complementing barrier properties to the final film, such as for example films having a vapour deposited barrier layer and a further polymer-based barrier layer coated onto the same substrate film. Another alternative of barrier materials are obtainable from barrier polymers that are coated in the form of a dispersion or solution in a liquid or solvent, onto a substrate, and subsequently dried into thin barrier coatings. Examples of suitable polymers for aqueous compositions are polyvinyl alcohols (PVOH), water-dispersible ethylene vinyl alcohols (EVOH) or polysaccharide-based water-dispersible or dissolvable polymers. Such dispersion coated or so called liquid film coated (LFC) layers may be made very thin, down to tenths of a gram per m2, and may provide high quality, homogenous layers. Furthermore, such liquid film coated polymer layers from water-dispersible or -dissolvable polymers often provide good internal adhesion to adjacent layers, which contributes to good integrity of the final packaging container. With package integrity is generally meant the package durability, i.e. the resistance to leakage of a packaging container, but also the package barrier properties towards substances migrating through the package walls from the outside. Whatever the layer combination in a laminated packaging material to be suitable for liquid or wet food packaging, the package integrity is a very important factor for protecting the packaged food content in satisfactory manner. A main contributing property to package integrity is the adhesion between layers of the laminated packaging material, and weaknesses in this respect may open up for ingress of harmful substances in a formed, filled and sealed package, which is subsequently handled in transporting and distribution to retailers, and even to leakage of the filled product to the outside. Even if the weaknesses in adhesion would not lead to such ingress or leakage, they may cause that the appearance of the package is deteriorated, in various respects. The packages may lose their grip stiffness, lose their distinct shape of a cuboid package such that they have bulging package walls, and unsharp edges and corners, and the printed décor may be damaged by moisture or air creeping in between the printed décor and the covering, protective polymer layer.
Another type of package that has high demands on package integrity is the package made from carton-based laminated packaging materials which are durable for sterilisation of filled packages in autoclave of retort treatment, intended for packaging of soups, pastes and semi-solid food as well as solid, wet food products.
The appearance of the above described packages is dependent on the printed décor of the packaging container. The printed décor, is conventionally applied by means of high-speed flexography processes or to a decreasing extent by off-set printing processes. These printing processes are designed for high-speed printing of wide substrate webs of several meters in e.g. packaging material manufacturing plants, and have in common that they require complex pre-printing set-up activities by the production of imaged “negative” printing plates that will transfer the ink and printing pattern to the substrate, in order to provide a desired décor for a certain product to be packed, and each design or colour change in the printed décor requires a change also in the pre-printing plate and print forme preparations.
A more flexible way of printing is the printing by ink-jet technology, according to which printing heads a jetting the ink onto the substrate in a controlled manner, by direct control by computer signals. This technology thus eliminates the whole pre-printing operations of preparing and mounting imaged printing plates onto printing forms, and is replaced by direct digital control of the printing operation. Hitherto, the ink-jet technology has not been suitable for the printing of wide-web substrates moving at high speeds, as is the case in the manufacturing of laminated packaging materials, but recent ink-jet technology developments have opened up for entering also into this area. Some research and development has been done in this respect of providing good image quality also at high speeds of various substrates, but the technology has still failed in the lamination of the printed substrates into laminated material structures, due to bad adhesion between the printed décor layer and the next layer of a coating or a film of a thermoplastic polymer in the laminated material. The bad adhesion between the printed décor layer and the outermost, protective, thermoplastic polymer layer or film of a packaging laminate would have bearing also on the above described property of package integrity, since the packaging material for a liquid or wet food packaging container of the form-fill-seal type, could delaminate and in particular destroy the printed décor appearance towards the outside and the customers and consumers, in addition to also creating risk areas for ingress or leakage around openings and sensitive areas of the formed, filled and sealed package.
For various reasons of image and print quality of the ink-jet printing technology, it has proven necessary to pre-treat the substrate surface with a pre-coating solution and optionally drying it, before printing with the ink-jet ink.
Inkjet inks are characterized by low viscosity, low solids, and aqueous solvent. When glossy or semi-glossy papers are printed with inkjet inks that comprise as much as 90-95% water as the carrier solvent, the inks have a tendency to sit on the surface of the coating, rather than penetrate into the coating and/or underlying paper substrate.
Because the inks printed on a water-resistant receiver dry primarily by evaporation of the water without any significant penetration or absorption of the water into the coating or paper, a number of problems are encountered. One such problem is that the individual ink droplets slowly spread laterally across the surface of the coating, eventually touching and coalescing with adjacent ink droplets. This gives rise to a visual image quality artifact known as “coalescence” or puddling. Another problem encountered when inks dry too slowly is that when two different color inks are printed next to each other, such as when black text is highlighted or surrounded by yellow ink, the two colors tend to bleed into one another, resulting in a defect known as “intercolor bleed.” Yet another problem is that when printing at high speed, either in a sheet fed printing process, or in a roll-to-roll printing process, the printed image is not dried sufficiently before the printed image comes in contact with an unprinted surface, and ink is transferred from the printed area to the unprinted surface, resulting in “ink retransfer”.
In contrast to glossy papers, some unlaminated papers have matte surfaces that are very porous. In particular, the uncoated, unlaminated liquid paperboards for use in laminated liquid packaging materials, have a porous and a highly absorbing print surface. In this case, the colorant of aqueous inkjet inks tends to absorb deeply into the paper, resulting in a substantial loss of optical density and as a consequence, reduced color gamut.
High speed continuous inkjet printing processes have been developed that are suitable for high speed, mid-volume printing and have become of interest to the commercial printing industry. The requirements of commercial printing industry include, among others, image quality in terms of high optical density, broad color gamut, sharp detail, and minimal problems with coalescence, smearing, feathering and the like. Operationally, the printing process strives for low environmental impact, low energy consumption, fast drying, and so forth. The resulting print exhibits durability, resisting abrasion when dry or if wetted.
A surface treatment of a glossy, matte or porous paper substrate is thus necessary to fix the colorant pigments or the inkjet ink to the substrate surface and to enable high-quality inkjet printing. Untreated, i.e. not pre-coated, paper does not maintain the ink colorant at the surface, but allows significant penetration of the colorant into the interior of the paper, resulting in a loss of optical density and a low-quality image. Moreover, ink penetrates non-uniformly into the paper due to the heterogeneous nature of the paper, giving rise to mottle, which further degrades the image.
However, when further attempted to laminate the printed substrate surface to a further layer or a film of a thermoplastic polymer, it has been seen that the adhesion is insufficient between the printed ink and the outermost protective thermoplastic polymer layer. Such an outermost protective thermoplastic polymer layer is necessary in e.g. liquid food packaging, to proved liquid barrier towards wet conditions on the outside of the package and to provide heat sealability in the form-fill-seal packaging process.